Well I took the leap into replacing my fuel lines and looking for the overfill leak. What if found is quite interesting.

Here is my story!

The fill lines are a GM part number, non wire re-enforced, and the old supply lines were hard and brittle. The Vent line 5/8 to gas fill was the worst. Found an old nest on top of the tank as well!
Here is the lines going into tank from fill. The fill on our 310 was a solid alum. pipe from fill to bottom hose connection.
The tank was listed as "For Diesel Fuel Only"
That must be the reason for the sediment trap on the bottom of the tank?!
Here is what came out on the drain out. I drained out 16 gal. and that is when the Gen-set would not draw any longer.
Here is a look at the hoses coming out of the frame and you can see the shape of the 5/8 hose after I cut it. The outer covering was just falling off.
To be continued:

On the 310, you will have to cut an access hole to get the back strap off. This is on the left electrical cord/air compressor compartment.
Here is what I found on top of the tank!
The fill line was far from being what I would expect! You do not clamp on a pipe thread and expect it to not leak.
Here is the fix for clamping on the threads. I turned down a pipe coupler to 1 1/2 dia. and sealed it on the pipe. Then double clamped it to the new hose.
Here is a cleaned up look at the top of the tank. The center protected fitting is the roll over vent valve. There is a small brass ball in the neck, and if you roll the rig over, it should keep gas from coming out of the tank. The only problem is that is has no filter on it, and if over filled will run out and down the tank under the coach!!
Interesting find on the tank, did Henschen do Airstreams prep work on the frame??
Here is my new fill hose. Double wire re-enforced!
Here is the tank ready to install. The center roll over vent now has a 5/16 line that will vent to the outside. (update to follow)
The tank had 3 extra fittings, and they had just soldered a reducer. They were seeping and I replaced them with real plugs!
To be continued:

Here is the top of the tank with an extra ground in place and new real plugs in the extra tank fittings.
Here is some shots of the hoses, fill and vent with the tank in place. You can see the 5/16 vent hose from the roll over check valve. I drilled a hole in the front floor support area just above the frame to route the new 5/16 vent line to keep it high as possible.
Here is the tank in-place. I did it all my self, but would recommend that you have a helper to keep from crushing the fill line. The line is crush resistant, not crush proof.
Here is the access hole covered to get to the back strap nut.
Here is the pile of hose that came off the tank.
Here is the tag that shows the tank is to be used for Diesel only!

Some points to ponder:
I used a floor jack to drop the tank. The drain plug made it much easer to get the fuel (gas) out. I put 6 gal. back in the tank and was able to make it to fill it back up. Almost $200 worth!!
I used the fuel injection hose were possible and the clamps as well. Double clamped the fill line at both places, tank and fill. The vent I double clamped at the tank and single at the fill.
The roll over vent line will be addressed at a later post. I have something up my sleeve for that and will show it to y'all after I get it installed.
If you have any questions, I will be glad to help anyway I can.

Very interesting Chris, and as usual a very thorough coverage of your excellent workmanship. BUT.......
You have now made me very nervous!!! I intended to clear out the tank at some time, and as my tank looks the same, I now have a very good idea what to expect.
We have been advised here to change all flexible hoses due to the new formula for unleaded gas, which rots out the old fuel lines. I have just changed the ones "up front" but the tank will HAVE to be next!

We have 10% ethanol in most of our petrol here in the states. My fuel lines were hard and small cracks on the outside, but the inside looked intact. I'm sure 30 years has the most effect on them, but the ethanol will not help in the future I'm sure.

Remember that we have the filter forward on the rear axle that has hoses on it. Make sure you change that as well.

Excellent write up. I vote this thread to be added to the The Airstream Classic Motorhome Sticky Thread

Done!

Thanks for the detailed writeup and pics!

I find that lump on the bottom of your tank scary!
Too easy for that to get hit!

My 345 tank is smooth underneath, and whilst I have not dropped it yet, here is a couple of pointers either for you or others following on..

The fuel mix these days is killer on older lines...
Here is what I found when I replaced the main chassis to fuel pump line under the engine...
It was not leaking until I touched it!

There are several flex line sections along the chassis rail too that should be checked and replaced.
Like this aprox under the entrance door, but there are 2 others I can see!
Note the signs of weepage at the larger feed pipe and the smaller return pipe joints!

I could always smell gas in the bedroom of my 345, and here is what I found...
Firstly, when I was filling the tank I saw a leak underneath...
The filler neck pipe was perished... you can see the stain.

This pipe on my 345 is 1 3/4" and you need about 24". The breather pipe is 3/8" I seem to remember.

I could still smell gas after I replaced it...

From the gas filler neck on the outside of the M/H there is a metal box/pipe assembly.
Inside the M/H it changes to a pair of flexible pipes... This is hidden behind an ABS cover under the bed.

Under the cover, it looks like this.
Note the leakage stains on the lower side of the filler pipe!

The filler pipe is 2" and the vent is 3/4"... you need about 6" of each.
This then goes back into a hard pipe below the floor...
Here is how that looks...

I got my new vent installed today. Not bad to do, and do not have gas fumes trapped under the coach any longer!

Here is what I used from Attwood marine supply. Just a habit from being around boats for over 40 years.
Here it is broke down.
The P-trap vent is set up for a 5/8 hose. I cut that off and taped the unit for 1/4 pipe tap so that I could put a 5/16 barb fitting for the line coming from my Turn over protection check valve.
I used a knock out punch of just under 1 3/8" dia., this is the size for a 1" pipe fitting. The unit as you can see is mounted in the top right hand corner of the left wheel well. All sealed up and with the 5/16 hose attached.
You can see the line with the coupler in place. I did not leave the hose long enough to do the connection with out it! You need 7' of hose to make it with out the coupler.
I place the corrugated cover on the hose for extra protection. There is a gap between the house frame to the outer skin that I was able to route the hose. Not to worry about a blow out taking out the line and vent trap, I will be placing a steal protection seal to cover up the whole set up.
And here is a look at the finished install. It is all stainless and as you saw the center section can be taken out to clean the screen if needed.

I was going to route the vent line into the gas fill door, but I did not want to tackle the removal of the protection cover in the coach, water tank and bed to get there! If I had the coupler joint like Keyair, I would be at a place where I would be doing that. The outside vent has a place for a 7/16 allen wrench so that I could tuck the vent right in the corner of the wheel well. This keeps the vent right at about the same hight of the gas fill.
I will protect the vent and line with a steel plate if there is a blow out, that my TPMS does not catch for me, the setup will be protected.
This will take care of my overfill leaking problems, and keep the gas fumes from venting under the coach.
Thanks everyone for looking at my project, and hope that there is something that will help you out.